Among this summer's Olympics and jubilees, Britons will celebrate the last time a British army actually won a war, 30 years ago on the Falklands. That is appropriate. But they are unlikely to read how it was really won. As in so many conflicts, victory lay not with the talents of the winners but with the mistakes of the losers. It was Argentina's mistake three decades ago this week that truly decided the outcome. Had it not been for the South Georgia "incident", wrecking Argentina's original invasion plan, Britain would probably have lost the Falklands. With them would have gone Margaret Thatcher's already battered political career.

Everything London said or did in the 1970s suggested a loss of interest in relics of empire. Aden, Hong Kong, Diego Garcia, Rhodesia, had gone or were going. The fleet was retreating to base. In the south Atlantic, Southern Thule had been surrendered to Argentina in 1976. The Antarctic survey ship HMS Endurance was to be withdrawn in April 1982. British diplomats were discussing a possible "transfer and leaseback" of the Falklands with Argentina at the UN.

In December 1981 a new Buenos Aires junta under General Galtieri understandably thought it a good moment to dust off a plan to seize the Falklands and its uninhabited dependency South Georgia. May to July 1982, with Endurance gone and the southern winter at full blow, was the preferred window. The UN could hardly object, having conceded India's military seizure of Portuguese Goa in 1961. Indeed, an early plan for the Falklands invasion was dubbed Operation Goa. Most eager for action was Argentina's navy, keen to cleanse its image after the tortures and killings of the "dirty war".

An initial plan was for the covert occupation of South Georgia by marine units under the cover of a compliant scrap-metal dealer, Constantino Davidoff. This was duly reconnoitred. But the junta soon decided to go for a full Falklands invasion, possibly as early as May. Planning was given to an able navy commander, Vice Admiral Lombardo, who insisted that if the Falklands were the objective, an earlier occupation of South Georgia be cancelled. It would lose him the virtue of surprise and risk Britain reinforcing the islands. His junta boss agreed and promised to cancel the South Georgia operation.

Hence 20 March 1982 found Lombardo on a beach in Uruguay, where he was horrified to read in the international press that the scrap merchants had landed on South Georgia. Lombardo was even more appalled to learn that Davidoff had been reinforced with a detachment of marines under Lieutenant Alfredo Astiz (later to be given a life sentence for dirty war atrocities). It was Astiz's gung-ho desire for celebrity that led a flag to be raised and the British alerted to the occupation as more than a metal forage.

Lombardo raced home furious at being double-crossed by the shadier elements of his own service. He felt Britain was now certain to react by sending a submarine and reinforcements. The junta's response was to order him to invade at once. This was despite assets being in the wrong place. Troops had not been allocated or trained. There were too few helicopters. A flotilla had to be conspicuously detached from exercises and loaded with conscripts under a junior commander. There would be little chance to arrange prior diplomatic cover, as advised by the equally dismayed foreign minister. A good plan at the right time was replaced by a bad one at the wrong time.

Anyone now reading back over London's response to these events can only gasp at their "cognitive dissonance". No one in Buenos Aires or London wanted to hear what did not suit their preconceptions. Most Britons treated the South Georgia landing as some sort of Ruritanian prank. A Thatcher cabinet drowned in the hysteria of cuts decided nothing serious would ensue, a failure for which the subsequent Franks committee absurdly exonerated it. When on 20 March Endurance was ordered from Port Stanley to South Georgia, it was too late, removing the one deterrent to an Argentine landing and leaving the Falklands exposed to attack. Many Argentinians were later convinced Thatcher had drawn them on to her punch to rescue her faltering leadership.

Lombardo's cobbled-together invasion took place on 2 April, leaving Thatcher initially stunned and humiliated. Though bloodless, its crudity aroused enough sympathy to engage President Reagan's public neutrality and private support. A British task force defied all doubters and sailed south, though cautious military opinion warned of its vulnerability to Argentine bombers and missiles. Most observers expected it to do no more than force a diplomatic compromise, which might have been the case had the US not given Britain covert help. Though the task force lost most of its helicopters, Argentina's conscript army offered only limited resistance after the task force landed. Lombardo's plan failed.

The South Georgia incident pre-empted what should have been a successful Argentine takeover of the Falklands. By May or June it is barely conceivable a task force would have sailed and by the following southern summer the occupation would have been a fait accompli. South Georgia arose from rivalries within the Argentine navy, putting buccaneering recklessness ahead of careful strategy. Thatcher had left British soil undefended and escaped a classic intelligence failure scot-free. She certainly owed a debt to her armed forces. But special thanks should go to the madcap merchant Davidoff, to the odious Astiz and to the entire Argentine navy. Without them, there would have been no Thatcherism and no Oscar this year for Meryl Streep.